Fiber lasers are typically lasers with optical fibers as gain media, although some lasers with a semiconductor gain medium and a fiber cavity have also been called fiber lasers. In most cases, the gain medium is a fiber doped with rare-earth ions such as erbium, neodymium, ytterbium, or thulium, and one or several laser diodes are used for pumping. Some benefits associated with fiber lasers include: a large gain bandwidth due to strongly broadened laser transitions in glasses, enabling wide wavelength tuning ranges and/or the generation of ultra-short pulses, the potential for very high output powers (e.g., several kilowatts with double-clad fibers) due to a high surface-to-volume ratio (avoiding excessive heating) and the guiding effect, which avoids thermo-optical problems even under conditions of significant heating just to name a few.
Additional benefits can accrue in fiber lasers that have a modified gain medium or, in the case of a fiber laser, a modified gain fiber. As disclosed above, fiber lasers often contain certain glasses such as silica or silica doped with germanium, or crystals such as Nd:YAG (i.e., neodymium-doped yttrium aluminum garnet), Yb:YAG (i.e., yttrium-doped YAG), Yb:glass, or Ti:sapphire, in the form of solid pieces or optical glass fibers. These fibers are doped with some active stimulated-emission ions (also called amplifying or laser ions), that in most cases are trivalent rare-earth ions, and which are optically pumped. The doping density of crystals and glasses often has to be carefully optimized. A high doping density may be desirable for good pump absorption in a short length, but may lead to energy losses related to quenching processes, (e.g. caused by clustering of laser-active ions and energy transport to defects).
In addition to doping the density of a gain fiber, additional energy can be generated by addressing issues related to the refractive index of the core and the cladding material. More to the point, the energy generated by a fiber laser is also dependent upon the various refractive and modal index values associated with a particular gain-fiber configuration. In some configurations, a gain fiber will be configured such that its refractive index is a step index value. The step index fiber is the simplest case of a standard gain fiber.
Despite the benefits associated with fiber lasers, there are problems associated with these types of lasers. For example, complicated temperature-dependent polarization evolution, the various nonlinear effects of which often limit the performance, and risk of fiber damage at high powers (commonly known as “fiber fuse”). When fiber fuse occurs, the fiber can burn down starting from the output end and propagating back towards the input end.
The problems of temperature-dependent polarization and fiber fuse become even more acute, given the current and proposed uses of fiber lasers. As described above, fiber lasers can be used to produce very high output powers. Given these high output powers, fiber lasers have uses for military and industrial applications requiring large amounts of energy. Accordingly, it is necessary to develop methods and apparatus to enable high energy to be used in conjunction with fiber lasers, while at the same time avoiding the aforementioned problems of, for example, fiber fuse.